Multiple-piece golf ball

ABSTRACT

A multiple-piece golf ball in accordance with the present invention includes a core located in the center of the golf ball, at least two intermediate layers surrounding the core, and a cover further surrounding the intermediate layers. The golf ball has a spherical zone having a bulk specific gravity of about 0.7 or less, the spherical zone having a radius of 18.5 mm with the same center point as the golf ball. The moment of inertia of this golf ball is preferably about 85 g·cm 2  or more.

BACKGROUND OF THE INVENTION

The present invention relates to a multiple-piece golf ball.

There are many factors influencing the carry of a golf ball, and among these factors, three factors of ball initial velocity, delivery angle, and spin speed are regarded as very important. Among these three factors, the ball spin is the essential factor for raising the golf ball and increasing the carry. However, if the spin speed is too high, the golf ball will be blown upward, or the run of the ball after falling decreases, which prevents the carry from increasing. The spin speed is the highest at the early stage at which the golf ball is delivered, and decreases gradually, that is, slows down during the flight of the golf ball.

Japanese Patent Application Publication No. 10-127815 discloses a golf ball having a moment of inertia of 81 to 86 g·cm², including a hollow core consisting of a hollow part having a 5 to 25 mm diameter and a core outer layer part having a specific gravity of about 1.2 to 1.9 surrounding the hollow part. This Publication also discloses that the core of a two-piece solid golf ball is made of two layers, and the specific gravity of the inner layer hull is made small and that of the outer layer hull is made large, whereby the moment of inertia of the golf ball is increased.

Japanese Patent Application Publication No. 11-70189 discloses a golf ball including a hollow part having a diameter of about 9 to 25 mm, a hollow core consisting of a material having a specific gravity of 1.05 to 1.25, and a 1 to 3-mm thick resin layer consisting of a material having an Izod impact strength of 50 J/m or higher, formed on the inner surface of the hollow core. This Publication also discloses that although the increase in the hollow part increases the moment of inertia, the hollow core may be easily broken by an impact force applied to the ball at the impact time; however, the formation of the resin layer can prevent the hollow core from being broken.

SUMMARY OF THE INVENTION

The above-described Publications disclose that the moment of inertia of the golf ball is increased to increase the carry of the golf ball, and also disclose that to increase the moment of inertia, the core is caused to have a hollow part having the largest possible diameter. However, if the diameter of the hollow part is increased, there arises a problem of decreased durability of the core and the golf ball. Also, if a material having a large specific gravity, which provides a high strength, is used for a core layer surrounding the hollow part to maintain the durability, the specific gravity on the center side of golf ball becomes large, so that the moment of inertia is decreased, which poses a problem that the carry cannot be increased sufficiently.

Accordingly, an object of the present invention is to provide a multiple-piece golf ball capable of increasing the moment of inertia of the golf ball significantly and thereby increasing the carry while the durability of the golf ball is maintained.

To achieve the above object, the present invention provides a multiple-piece golf ball including a core located in the center of the golf ball, at least two intermediate layers surrounding the core; and a cover further surrounding the intermediate layers, wherein the golf ball has a spherical zone having a bulk specific gravity of up to about 0.7, the spherical zone having a radius of 18.5 mm with the same center point as the golf ball.

The core may include a hollow part in the center thereof and a surrounding layer surrounding the hollow part. In this case, preferably, at least either one of the innermost intermediate layer of the at least two intermediate layers and the surrounding layer has a bulk specific gravity of about 0.8 or less. The innermost intermediate layer of the at least two intermediate layers is usually in the spherical zone. The hollow part preferably has a spherical shape having a diameter less than about 5 mm. The surrounding layer may be a foam or may have a space not filled with a material. Also, an intermediate layer located in the spherical zone of the at least two intermediate layers may be a foam or may have a space not filled with a material.

The core may be a foam in place of the above-described hollow core. Also, an intermediate layer located in the spherical zone of the at least two intermediate layers may be a foam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing one embodiment of a multiple-piece golf ball in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of a multiple-piece golf ball in accordance with the present invention will now be described with reference to the accompanying drawing. The present invention is not limited to this embodiment. The accompanying drawing is not drawn scaled down for better understanding of the present invention.

As shown in FIG. 1, a multiple-piece golf ball of this embodiment mainly includes a core 10 located in the central portion of the ball, intermediate layers 20 surrounding the outside of the core 10, and a cover 30 surrounding the outside of the intermediate layers 20. The design is made so that the bulk specific gravity of a spherical zone having a radius R of 18.5 mm with the center point C of the golf ball 1 being the center is very small, being about 0.7 or smaller. In this specification, the “bulk specific gravity” is represented by the ratio of the mass of a predetermined zone to the mass of 4° C. pure water. The whole interior of the zone need not be filled with a material, and a space such as a cavity or air bubbles may be present in the zone.

By this configuration, the center side of the golf ball 1 is made light and the outside thereof is made heavy, so that the moment of inertia of the golf ball 1 can be increased. Therefore, since the spin decay rate during flight decreases, even when the initial spin speed is low, the spin speed during flight is maintained in a proper range, so that the carry can be increased. The upper limit of bulk specific gravity of the above-described zone is preferably about 0.75, further preferably about 0.725. The lower limit of bulk specific gravity of the above-described zone is not subject to any special restriction, but is preferably about 0.2, more preferably about 0.3.

The moment of inertia of the golf ball 1 should be designed so as to be preferably about 85 g·cm² or higher, more preferably about 90 g·cm² or higher, and still more preferably about 95 g·cm² or higher. The upper limit of the moment of inertia of the golf ball 1 is not subject to any special restriction, but is preferably about 120 g·cm², more preferably about 118 g·cm², and still more preferably about 115 g·cm². Also, the spin decay rate should be designed so as to be preferably about 2.65% or lower, more preferably about 2.6% or lower. The lower limit of the spin decay rate is not subject to any special restriction, but is preferably about 2.0%, more preferably about 2.2%.

Hereunder, the constructions of the core 10, the intermediate layers 20, and the cover 30 of the golf ball 1 having the above-described configuration are explained in detail.

The core 10 includes a hollow part 10 a located in the central portion of the core 10 and a surrounding layer 10 b surrounding the outside of the hollow part 10 a. The hollow part 10 a substantially has a spherical shape. The surrounding layer 10 b substantially has a spherical external shape.

The lower limit of the diameter of the hollow part 10 a is preferably about 2 mm, more preferably about 3 mm. On the other hand, the upper limit of the diameter of the hollow part 10 a is preferably about 5 mm, more preferably about 4 mm. The lower limit of the outside diameter of the core 10 is preferably about 6 mm, more preferably about 7 mm. The upper limit of the outside diameter of the core 10 is preferably about 35 mm, more preferably about 34 mm. The lower limit of the thickness of the surrounding layer 10 b is preferably about 2 mm, more preferably about 2.5 mm. The upper limit of the thickness of the surrounding layer 10 b is preferably about 18 mm, more preferably about 17 mm.

The lower limit of the bulk specific gravity of the surrounding layer 10 b is preferably about 0.2, more preferably about 0.3, and still more preferably about 0.4. The upper limit of the bulk specific gravity of the surrounding layer 10 b is preferably about 4.0, more preferably about 3.8, and still more preferably about 3.6. It is preferable that the bulk specific gravity of either one of the surrounding layer 10 b and the later-described first intermediate layer 20 a be made 0.8 or smaller.

As a material forming the core 10 or the surrounding layer 10 b, for example, a rubber composition containing a base rubber, being a principal component, and optionally containing a co-crosslinking agent, an initiator, a filler, a foaming agent, an antioxidant, and an organo-sulfur compound can be used. Also, as the principal component, in place of the base rubber, a thermoplastic elastomer, an ionomer resin, or a mixture thereof can be used.

As the base rubber, a thermosetting elastomer can be used widely, and, for example, polybutadiene rubber (BR), styrene-butadiene rubber (SBR), natural rubber (NR), polyisoprene rubber (IR), polyurethane rubber (PU), butyl rubber (IIR), vinyl polybutadiene rubber (VBR), ethylene-propylene rubber (EPDM), nitrile rubber (NBR), and silicone rubber can be used. However, the base rubber is not limited to these rubbers. As the polybutadiene rubber (BR), for example, 1,2-polybutadiene, c is 1,4-polybutadiene, and the like can be used.

Polybutadiene preferably has a Mooney viscosity (ML₁₊₄ (100° C.)) of about 30 or higher. The Mooney viscosity thereof is more preferably about 35 or higher, still more preferably about 40 or higher, yet still more preferably about 50 or higher, and most preferably about 52 or higher. The upper limit of the Mooney viscosity thereof is preferably about 100, more preferably about 80, still more preferably about 70, and yet still more preferably about 60.

The Mooney viscosity in the present invention is an industrial viscosity index (JIS-K6300) measured by a Mooney viscometer, which is one kind of rotary plasticity meter, and as the unit symbol thereof, ML₁₊₄ (100° C.) is used. In this unit symbol, M denotes Mooney viscosity, L denotes a large rotor (L type), 1+4 denotes that the preheating time is one minute and the rotor rotating time is four minutes, and 100° C. denotes that measurement is made under a condition of 100° C.

Furthermore, the molecular weight distribution Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight) of polybutadiene is preferably about 2.0 or more, more preferably about 2.2 or more, still more preferably about 2.4 or more, and yet still more preferably about 2.6 or more. The upper limit of the molecular weight distribution thereof is preferably about 6.0, more preferably about 5.0, still more preferably about 4.0, and yet still more preferably about 3.4. If Mw/Mn is too low, the workability may deteriorate, and if Mw/Mn is high, the resilience performance may degrade.

Although polybutadiene may be synthesized by using a Ni or Co catalyst, or may be synthesized by using a rare earth element based catalyst; in particular, polybutadiene is preferably synthesized by using a rare earth element based catalyst. As the rare earth element based catalyst, a publicly known rare earth element based catalyst can be used. For example, a lanthanoid rare earth element based compound, an organic aluminum compound, alumoxane, and a halogen-containing compound, and further, a catalyst consisting of a combination of Lewis base as necessary can be cited.

In the present invention, in particular, the use of a neodymium based catalyst using a neodymium compound as the lanthanoid rare earth element compound is preferable because polybutadiene rubber having a high content of 1,4-cis bond and a low content of 1,2-vinyl bond is obtained with excellent polymerization activity. As the specific examples of the rare earth element based catalyst, the catalysts described in Japanese Unexamined Patent Application Publication No. 11-35633 can be cited preferably, the citation thereof herein forming a part of description of this specification.

In the case in which butadiene is polymerized in the presence of the rare earth element based catalyst, a solvent may be used, or bulk polymerization or vapor-phase polymerization may be made without the use of solvent. The polymerization temperature can be made usually about −30° C. to about 150° C., preferably about 10 to 100° C.

The above-described polybutadiene may be obtained by allowing a terminal modifying agent to react with the active terminal of polymer following the polymerization using the rare earth element based catalyst. As the specific examples and the reaction methods of the terminal modifying agent, for example, the terminal modifying agents and the reaction methods described in Japanese Unexamined Patent Application Publication Nos. 11-35633, 07-268132, and 2002-293996 can be cited, the citation thereof herein forming a part of description of this specification.

Polybutadiene is blended in the rubber base material preferably in an amount of about 60 wt % or more, more preferably in the amount of about 70 wt % or more, still more preferably in the amount of about 80 wt % or more, and most preferably in the amount of about 90 wt % or more. The upper limit of the blend ratio of polybutadiene is preferably about 100 wt %. By blending polybutadiene within this range, a golf ball having satisfactory resilience performance can be obtained.

Also, the aforementioned rubber other than polybutadiene can be blended, in addition to polybutadiene, in a range so as not to compromise the object of the present invention. The rubbers especially preferable for being blended in addition to polybutadiene are styrene-butadiene rubber, natural rubber, polyisoprene rubber, ethylene-propylene-diene rubber, and the like. These rubbers can be used singly in one kind, or can be used by combining two or more kinds.

As the co-crosslinking agent, although not limited thereto, for example, α,β-unsaturated carboxylic acid or a metallic salt thereof can preferably be used. As the α,β-unsaturated carboxylic acid or the metallic salt thereof, for example, acrylic acid, methacrylic acid, and a zinc salt, magnesium salt, calcium salt, and the like thereof can be cited. As the blending amount of the co-crosslinking agent, although not limited to this, for example, with respect to 100 weight parts of rubber base material, about 5 weight parts or more is preferable, and about 10 weight parts or more is more preferable. Also, the blending amount of the co-crosslinking agent is preferably about 70 weight parts or less, more preferably about 50 weight parts or less.

As the initiator, although not limited to this, an organic peroxide is preferably used. As the blending amount of the initiator, although not limited to this, for example, with respect to 100 weight parts of rubber base material, about 0.10 weight part or more is preferable, about 0.15 weight part or more is more preferable, and about 0.30 weight part or more is still more preferable. Also, the blending amount of the initiator is preferably about 8 weight parts or less, more preferably about 6 weight parts or less.

As the filler, for example, silver, gold, cobalt, chromium, copper, iron, germanium, manganese, molybdenum, nickel, lead, platinum, tin, titanium, tungsten, zinc, zirconium, barium sulfate, zinc oxide, manganese oxide, and the like can be used, but the filler is not limited to these. The filler is preferably in a powder form. As the blending amount of the filler, although not limited to this, for example, with respect to 100 weight parts of rubber base material, about 2 weight parts or more is preferable, about 5 weight parts or more is more preferable, and about 10 weight parts or more is still more preferable. Also, the blending amount of the filler is preferably about 1000 weight parts or less, more preferably about 900 weight parts or less, and still more preferably about 800 weight parts or less.

As the foaming agent, although not limited to this, for example, azodicarbonamide, azobisisobutyronitrile, dinitrosopentamethylenetetramine, p-toluenesulfonylhydrazide, p,p′-oxybis (benzenesulfonylhydrazide), and sodium hydrogencarbonate, can be used. As the blending amount of the foaming agent, although not limited to this, for example, with respect to 100 weight parts of rubber base material, about 2 weight parts or more is preferable, and about 3 weight parts or more is more preferable. Also, the blending amount of the foaming agent is preferably about 30 weight parts or less, more preferably about 25 weight parts or less.

Although FIG. 1 shows the core 10 the central portion of which is hollow, the present invention is not limited to this configuration, and a solid core may be used. Also, the core 10 or the surrounding layer 10 a may be formed so as to have a space that is not filled with a material. Such a space, for example, may be a concave-shaped space formed by depressing a part of a layer surface, or may be a hole-shaped space penetrating the layer. Alternatively, a space can be provided in the core 10 or the surrounding layer 10 a by adding a foaming agent to the material of the core 10 or the surrounding layer 10 a. As the foaming agent, the above-described foaming agent can be used. As the blending amount of the foaming agent, although not limited to this, for example, with respect to 100 weight parts of principal component, about 2 weight parts or more is preferable, and about 3 weight parts or more is more preferable. Also, the blending amount of the foaming agent is preferably about 30 weight parts or less, more preferably about 25 weight parts or less.

The intermediate layers 20 include the first intermediate layer 20 a on the center side and a second intermediate layer 20 b on the outside. The distance from the center C of the golf ball 1 to the outside surface of the first intermediate layer 20 a is a distance D of 18.5 mm as described above. The lower limit of the thickness of the first intermediate layer 20 a is preferably about 0.5 mm, more preferably about 1.0 mm, and still more preferably about 1.5 mm. The upper limit of the thickness of the first intermediate layer 20 a is preferably about 16 mm, more preferably about 15 mm, and still more preferably about 14 mm.

The bulk specific gravity of the first intermediate layer 20 a is preferably about 0.2 or larger, more preferably about 0.3 or larger, and still more preferably about 0.4 or larger. The upper limit of the bulk specific gravity of the first intermediate layer 20 a is preferably about 3.8, more preferably about 3.6. It is preferable that the bulk specific gravity or either one of the surrounding layer 10 b and the first intermediate layer 20 a be made 0.8 or smaller.

The lower limit of the thickness of the second intermediate layer 20 b is preferably about 0.5 mm, more preferably about 0.8 mm, and still more preferably about 1.0 mm. The upper limit of the thickness of the second intermediate layer 20 b is preferably about 16 mm, more preferably about 15 mm, and still more preferably about 14 mm.

The bulk specific gravity of the second intermediate layer 20 b is preferably about 0.2 or greater, more preferably about 0.3 or greater, and still more preferably about 0.4 or greater. The upper limit of the bulk specific gravity of the second intermediate layer 20 b is preferably about 3.8, and more preferably about 3.6.

As the material for the first and second intermediate layers 20 a and 20 b, although not limited to this, a thermosetting elastomer, a thermoplastic elastomer, an ionomer resin, or a mixture thereof can be used.

As the thermosetting elastomer, although not limited to this, polybutadiene rubber (BR), styrene-butadiene rubber (SBR), natural rubber (NR), polyisoprene rubber (IR), polyurethane rubber (PU), butyl rubber (IIR), vinyl polybutadiene rubber (VBR), ethylene-propylene rubber (EPDM), nitrile rubber (NBR), and silicone rubber can be used.

Polybutadiene preferably has a Mooney viscosity (ML₁₊₄ (100° C.)) of about 30 or higher. The Mooney viscosity thereof is more preferably about 35 or higher, still more preferably about 40 or higher, yet still more preferably about 50 or higher, and most preferably about 52 or higher. The upper limit of the Mooney viscosity thereof is preferably about 100, more preferably about 80, still more preferably about 70, and yet still more preferably about 60.

The Mooney viscosity in the present invention is an industrial viscosity index (JIS-K6300) measured by a Mooney viscometer, which is one kind of rotary plasticity meters, and as the unit symbol thereof, ML₁₊₄ (100° C.) is used. In this unit symbol, M denotes Mooney viscosity, L denotes a large rotor (L type), 1+4 denotes that the preheating time is one minute and the rotor rotating time is four minutes, and 100° C. denotes that measurement is made under conditions of 100° C.

Furthermore, the molecular weight distribution Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight) of polybutadiene is preferably about 2.0 or more, more preferably about 2.2 or more, still more preferably about 2.4 or more, and yet still more preferably about 2.6 or more. The upper limit of the molecular weight distribution thereof is preferably about 6.0, more preferably about 5.0, still more preferably about 4.0, and yet still more preferably about 3.4. If Mw/Mn is too low, the workability may deteriorate, and if Mw/Mn is too high, the resilience performance may degrade.

Although polybutadiene may be synthesized by using a Ni or Co catalyst, or may be synthesized by using a rare earth element based catalyst, in particular, polybutadiene is preferably synthesized by using a rare earth element based catalyst. As the rare earth element based catalyst, a publicly known rare earth element based catalyst can be used. For example, a lanthanoid rare earth element compound, an organic aluminum compound, alumoxane, and a halogen-containing compound, and furthermore a catalyst consisting of a combination of Lewis base as necessary can be cited.

In the present invention, in particular, the use of a neodymium based catalyst using a neodymium compound as the lanthanoid rare earth element compound is preferable because polybutadiene rubber having a high content of 1,4-cis bond and a low content of 1,2-vinyl bond is obtained with excellent polymerization activity. As the specific examples of the rare earth element based catalyst, the catalysts described in Japanese Unexamined Patent Application Publication No. 11-35633 can be cited preferably, the citation thereof herein forming a part of description of this specification.

In the case in which butadiene is polymerized in the presence of the rare earth element based catalyst, a solvent may be used, or bulk polymerization or vapor-phase polymerization may be made without the use of solvent. The polymerization temperature can be made usually about −30° C. to about 150° C., preferably about 10 to 100° C.

The above-described polybutadiene may be obtained by allowing a terminal modifying agent to react with the active terminal of polymer following the polymerization using the rare earth element based catalyst. As the specific examples and the reaction methods of the terminal modifying agent, for example, the terminal modifying agents and the reaction methods described in Japanese Unexamined Patent Applications Publications Nos. 11-35633, 07-268132, and 2002-293996 can be cited, the citation thereof herein forming a part of description of this specification.

Polybutadiene is blended in the rubber base material preferably in the amount of about 60 wt % or more, more preferably in the amount of about 70 wt % or more, still more preferably in the amount of about 80 wt % or more, and most preferably in the amount of about 90 wt % or more. The upper limit of the blend ratio of polybutadiene is preferably about 100 wt %. By blending polybutadiene within this range, a golf ball having satisfactory resilience performance can be obtained.

Also, the aforementioned rubber other than polybutadiene can be blended, in addition to polybutadiene, in a range that does not compromise the object of the present invention. The rubbers especially preferable for being blended in addition to polybutadiene are styrene-butadiene rubber, natural rubber, polyisoprene rubber, ethylene-propylene-diene rubber, and the like. These rubbers can be used alone as one kind, or can be used by combining two or more kinds.

As the thermoplastic elastomer, although not limited to this, for example, a polyester based thermoplastic elastomer, a polyurethane based thermoplastic elastomer, a polyamide based thermoplastic elastomer, and a polyolefin based thermoplastic elastomer can be used.

As the ionomer resin, although not limited to this, a resin using the following (a) component and/or (b) component as a base resin can be used. To the base resin, the following (c) component can be added optionally. The (a) component is a tertiary random copolymer of olefin-unsaturated carboxylic acid-unsaturated tertiary random copolymer of olefin-unsaturated carboxylic acid-unsaturated carboxylate carboxylate and/or a metallic salt thereof, the (b) component is a binary random copolymer of olefin-unsaturated carboxylic acid and/or a metallic salt thereof, and the (c) component is a thermoplastic block copolymer having a polyolefin crystal block polyethylene/butylene random copolymer.

The weight-average molecular weight (Mw) of the tertiary random copolymer of olefin-unsaturated carboxylic acid-unsaturated carboxylate and/or the metallic salt thereof constituting the (a) component is preferably about 100,000 or greater, more preferably about 110,000 or greater, and still more preferably about 120,000 or greater. The upper limit thereof is preferably about 200,000, more preferably about 190,000, and still more preferably about 170,000. Also, the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the copolymer is preferably about 3.0 to about 7.0.

The (a) component is a copolymer containing olefin, and as the olefin in the (a) component, for example, an olefin having a carbon number of 2 or more, the upper limit of which is 8 or less, especially 6 or less, can be mentioned. Specifically, ethylene, propylene, butene, pentene, hexene, heptene, and octene can be mentioned, and in particular, ethylene is preferable.

As the unsaturated carboxylic acid in the (a) component, for example, acrylic acid, methacrylic acid, maleic acid, and fumaric acid can be cited, and in particular, acrylic acid and methacrylic acid are preferable.

As the acid-unsaturated carboxylate in the (a) component, for example, a lower alkylester of the above-described carboxylic acid can be cited. Specifically, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate, and butyl acrylate can be cited, and in particular, butyl acrylate (n-butyl acrylate, i-butyl acrylate) is preferable.

The random copolymer of the (a) component can be obtained by random copolymerizing the above-described components according to the publicly known method. Herein, the content (acid content) of unsaturated carboxylic acid contained in the random copolymer is usually about 2 wt % or higher, preferably about 6 wt % or higher, and more preferably about 8 wt % or higher. The upper limit thereof is about 25 wt %, preferably about 20 wt %, and more preferably about 15 wt %. If the acid content is low, the resilience performance may degrade, and if the acid content is high, the workability of the material may deteriorate.

The metallic salt of the copolymer of the (a) component can be obtained by partially neutralizing the acid radical in the random copolymer of the above-described (a) component with metal ion.

Herein, as the metal ion for neutralizing the acid radical, for example, ions of Na, K, Li, Zn, Cu, Mg, Ca, Co, Ni and Pb can be cited, and among these, ions of Na, Li, Zn, Mg and Ca are preferably used, and more preferably, Zn ion is recommended. The degree of neutralization of random copolymer of these ions is not subject to any special restriction; however, being usually about 5 mol % or more, preferably about 10 mol % or more, and especially about 20 mol % or more. The upper limit thereof is usually about 95 mol %, preferably about 90 mol %, and especially about 80 mol %. If the degree of neutralization exceeds about 95 mol %, the formability may deteriorate. If it is lower than about 5 mol %, the addition amount of the inorganic metallic compound of an (e) component must be increased, which may be disadvantageous in terms of cost. Such a neutralizer can be obtained by a publicly known method, and, for example, can be obtained by introducing a compound of formate, acetate, nitrate, carbonate, hydrogencarbonate, oxide, hydroxide, and alkoxide of the metal ion to the random copolymer.

As the tertiary random copolymer of olefin-unsaturated carboxylic acid-unsaturated carboxylate constituting the (a) component, specifically, trade names “Nucrel AN4318”, “Nucrel AN4319”, “Nucrel AN4311” (manufactured by DuPont-Mitsui Polychemicals Co., Ltd.) and the like can be cited. Also, as the metallic salt of tertiary random copolymer of olefin-unsaturated carboxylic acid-unsaturated carboxylate, specifically, trade names “Himilan AM7316”, “Himilan AM7331”, “Himilan 1855”, “Himilan 1856” (manufactured by DuPont-Mitsui Polychemicals Co., Ltd.), trade names “Surlyn 6320”, “Surlyn 8120” (manufactured by DuPont U.S.A), and the like can be cited.

Also, the weight-average molecular weight (Mw) of the binary random copolymer of olefin-unsaturated carboxylic acid and/or a metallic salt thereof constituting the (b) component is preferably about 100,000 or greater, more preferably about 110,000 or greater, and still more preferably about 120,000 or greater. The upper limit thereof is preferably about 200,000, more preferably about 190,000, and still more preferably about 170,000. Also, the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the copolymer is preferably about 3.0 to about 7.0.

The ratio of the copolymer of the (b) component to the whole of the base resin is about 0 to about 20 wt %, and the lower limit thereof is preferably 1 wt %. The upper limit value thereof is preferably about 17 wt %, more preferably about 10 wt %, still more preferably about 8 wt %, and yet still more preferably about 5 wt %.

As a special example of the binary random copolymer of olefin-unsaturated carboxylic acid constituting the (b) component, trade names “Nucrel 1560”, “Nucrel 1525”, “Nucrel 1035”, and the like (manufactured by DuPont-Mitsui Polychemicals Co., Ltd.) can be cited. As the metallic salt of binary random copolymer of olefin-unsaturated carboxylic acid, specifically, trade names “Himilan 1605”, “Himilan 1601”, “Himilan 1557”, “Himilan 1705”, “Himilan 1706” (manufactured by DuPont-Mitsui Polychemicals Co., Ltd.), trade names “Surlyn 7930”, “Surlyn 7920” (manufactured by DuPont U.S.A), and the like can be cited.

As the thermoplastic, block copolymer having a polyolefin crystal block polyethylene/butylene random copolymer constituting the (c) component, for example, a copolymer having a crystalline polyethylene block (E) as a hard segment and a block consisting of a relatively random copolymer (EB) of ethylene and butylene as a soft segment can be cited, and a block copolymer having a structure of an E-EB system, E-EB-E system, and the like in which the hard segment lies at one terminal or both terminals as a molecular structure is preferably used.

The thermoplastic block copolymer having a polyolefin crystal block polyethylene/butylene random copolymer constituting the (c) component can be obtained, for example, by hydrogenating polybutadiene. As the polybutadiene used for hydrogenation, polybutadiene, in which as the bond mode in the butadiene structure, especially a 1,4-bond has a 1,4-polymerization part of about 95 to about 100 wt % in terms of a block, and the 1,4-bond in the total amount of butadiene structure is preferably about 50 to about 100 wt %, more preferably about 80 to about 100 wt %, is suitably used. That is, polybutadiene in which the 1,4-bond is preferably about 50 to about 100 wt %, more preferably about 80 to about 100 wt %, and which has a 1,4-bond part of about 95 to about 100 wt % in terms of a block is suitably used.

As the thermoplastic block copolymer of an E-EB-E system, a copolymer, in which both terminal parts of a molecular chain are 1,4-bond rich 1,4-polymerized substances, and the intermediate part is obtained by hydrogenating polybutadiene in which 1,4-bond and 1,2-bond are mixed, is suitable. Herein, the hydrogenation amount in hydrogenated substance of polybutadiene (the conversion ratio of the double bond in polybutadiene to the saturation bond) is preferably about 60 to about 100%, more preferably about 90 to about 100%. If the hydrogenation amount is too small, in a process of blending with ionomer resin or the like, gelation and the like may be deteriorated. Also, when the golf ball is formed, the intermediate layers may pose a problem of poor hitting durability.

In the block copolymer that is used suitably as the thermoplastic block copolymer, and has a structure of an E-EB system, E-EB-E system, in which the hard segment lies at one terminal or both terminals as a molecular structure, the amount of hard segment is preferably about 10 to about 50 wt %. If the amount of hard segment is too large, the object of the present invention is not achieved effectively in some cases, and if the amount of hard segment is too small, there arises a problem of poor formability of blended substance in some cases.

The melt index at a temperature of 230° C. and a test load of 21.2 N of the thermoplastic block copolymer is preferably about 0.01 to about 15 g/10 min, more preferably about 0.03 to about 10 g/10 min. If the melt index is out of the above-described range, a problem of welding, sinking, being short, and the like may occur. Also, the surface hardness of thermoplastic block copolymer is preferably 10 to 50. If the surface hardness is too low, the durability in repeated hitting of the golf ball may deteriorate. On the other hand, if the surface hardness is too high, the resilience performance of blended substance with ionomer resin may degrade. The number-average molecular weight of the thermoplastic block copolymer is preferably about 30,000 to about 800,000.

As the above-described thermoplastic block copolymer having a polyolefin crystal block polyethylene/butylene random copolymer, a commercially available product can be used, and as the copolymer, for example, Dynaron 6100P, 6200P, 6201B, and the like manufactured by Japan Synthetic Rubber Co., Ltd., can be cited. In particular, Dynaron 6100P is a block polymer having a crystalline olefin block at both terminals, and can be used suitably in the present invention. These olefin-based thermoplastic elastomers may be used singly in one kind, or can be used by combining two or more kinds.

Furthermore, for the material of the intermediate layers 20, with respect to 100 weight parts of the above-described resin components of (a) to (c) components, about 5 to about 100 weight parts of fatty acid having a molecular weight of about 280 to about 1500 or the derivative thereof can be mixed as a (d) component, and about 0.1 to about 10 weight parts of a basic inorganic metallic compound capable of neutralizing the acid radical in the (a), (b) and (d) components can be mixed as the (e) component.

The (d) component is a fatty acid having a molecular weight of about 280 to about 1500 or a derivative thereof, and is a component contributing to the improvement in the flowability of the heated mixture. Comparing with the (a) to (c) components, the (d) component has an extremely low molecular weight, so that it contributes to a remarkable increase in the melt viscosity of the mixture. Also, the fatty acid (or derivative) in the (d) component has a molecular weight not lower than about 280 and not higher than about 1500 and contains a high-content acid radical (or derivative), so that a loss of resilience performance due to addition is small.

The fatty acid or the derivative thereof of the (d) component may be an unsaturated fatty acid (derivative) containing a double bond or a triple bond in an alkyl group, or may be saturated fatty acid (derivative) in which the bond in the alkyl group is formed by a single bond only. The number of carbons in one molecule is usually about 18 or greater, and it is recommended that the upper limit of the number of carbons be about 80, especially about 40. If the number of carbons is small, the heat resistance deteriorates, and the content of acid radical becomes too high, so that desired flowability cannot be attained because of the interaction with the acid radical contained in the base resin. If the number of carbons is large, the flowability may decrease because of the increase in molecular weight, so that the (d) component may become difficult to use as a material.

As the fatty acid of the (d) component, specifically, stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid, linolic acid, linolenic acid, arachidic acid, lignoceric acid, and the like can be cited, and in particular, stearic acid, arachidic acid, behenic acid, and lignoceric acid can be used suitably.

Also, as the fatty acid derivative of the (d) component, a derivative in which a proton contained in the acid radical of fatty acid is substituted can be cited. As such a fatty acid derivative, a metallic soap substituted by metal ion can be cited as an example. As the metal ion used for the metallic soap, ions of, for example, Li, Ca, Mg, Zn, Mn, Al, Ni, Fe, Cu, Sn, Pb and Co can be cited, and in particular, ions of Ca, Mg and Zn are preferable. The Fe ion may be bivalent or trivalent.

As the fatty acid derivative of the (d) component, specifically, magnesium stearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate, zinc lignocerate, and the like can be cited, and in particular, magnesium stearate, calcium stearate, zinc stearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate, and zinc lignocerate can be used suitably.

As the blending amount of the (d) component, usually, with respect to 100 weight parts of the base resin, about 1 weight part or more is preferable. The upper limit of the blending amount thereof is usually about 100 weight parts, preferably about 90 weight parts, more preferably about 80 weight parts, and still more preferably about 70 weight parts.

When the above-described (a) and/or (b) component is used, publicly known metallic soap-denatured ionomer (U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760, WO 98/46671, and the like, the citation thereof herein forming a part of the description of this specification) can also be used.

The (e) component is a basic inorganic metallic compound capable of neutralizing the acid radical in the (a), (b) and (d) components. As described in the conventional example, when only the (a), (b) or (d) component, especially only metallic soap-denatured ionomer resin (for example, only the metallic soap-denatured ionomer resin described in the aforementioned Patent Publications) is heatedly mixed, fatty acid is yielded by exchange reaction of metallic soap with unneutralized acid radical contained in ionomer as shown below. This yielded fatty acid has low thermal stability, and vaporizes easily at the time of formation. Therefore, this fatty acid not only may cause poor formation but also may cause a remarkable decrease in paint film adhesion when the yielded fatty acid adheres to the surface of a formed product. The (e) component is blended to solve such a problem.

(1) Unneutralized acid radical contained in ionomer resin (2) Metallic soap (3) Fatty acid X: Metal cation

For the heated mixture used in the present invention, as described above, as the (e) component, the basic inorganic metallic compound capable of neutralizing the acid radical contained in the (a), (b) and (d) components is blended as an essential component. By blending the (e) component, the acid radical in the (a), (b) and (d) components is neutralized. Therefore, by the synergistic effect due to the blending of these components, the thermal stability of the heated mixture is increased, and at the same time, excellent formability is provided, which contributes to enhanced resilience performance as a golf ball.

The (e) component is a basic inorganic metallic compound capable of neutralizing the acid radical in the (a), (b) and (d) components, and it is recommended that the (e) component be preferably a monoxide or a hydroxide. Since having great reactivity with ionomer resin and containing no organic substance in the reaction byproduct, the (e) component can increase the degree of neutralization of heated mixture without impairing the thermal stability.

As the metal ion used for the basic inorganic metallic compound, for example, ions of Li, Na, K, Ca, Mg, Zn, Al, Ni, Fe, Cu, Mn, Sn, Pb, Co and the like can be cited. As the inorganic metallic compound, a basic inorganic filler containing these metal ions, specifically, magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide, lithium carbonate, and the like can be cited. As described above, a monoxide or a hydroxide is suitable, and preferably, magnesium oxide or calcium hydroxide that has great reactivity with ionomer resin can be used suitably.

The blending amount of the (e) component is usually about 0.1 to about 10 weight parts with respect to 100 weight parts of the base resin. The lower limit of the blending amount thereof is preferably about 0.5 weight part, more preferably about 1 weight part. The upper limit thereof is preferably about 5 weight parts, more preferably about 3 weight parts.

The heated mixture used in the present invention is obtained by blending the (a) to (e) components as described above, and is used to improve the thermal stability, formability, and resilience performance. For the heated mixture used in the present invention, it is recommended that about 70 mol % or more, preferably about 80 mol % or more, and more preferably about 90 mol % or more of acid radical in the mixture be neutralized. This high neutralization more reliably restrains the exchange reaction that presents a problem when only the above-described (a) and (b) components and fatty acid (derivative) are used, and can prevent fatty acid from being yielded. Therefore, there can be provided a material having remarkably increased thermal stability, excellent formability, and resilience performance remarkably upgraded as compared with the conventional ionomer resin.

Regarding the neutralization of the heated mixture of the present invention, to more reliably attain both the high degree of neutralization and the flowability, it is recommended that the acid radical of the heated mixture be neutralized by transition metal ion and alkali metal and/or alkaline-earth metal ion. Since the transition metal ion has a weaker ionic cohesive force than the alkali metal and/or alkaline-earth metal ion, some of acid radical in the heated mixture is neutralized, so that the flowability can be improved remarkably.

In the present invention, to the heated mixture, various addition agents can be further added as necessary; for example, a filler, pigment, dispersing agent, antioxidant, ultraviolet light absorbing agent, and light stabilizer can be added. Also, to improve the feeling at the hitting time, in addition to the above-described essential components, various nonionomer thermoplastic elastomers can be blended. As the nonionomer thermoplastic elastomer, for example, styrene based thermoplastic elastomer, ester based thermoplastic elastomer, and urethane based thermoplastic elastomer can be cited, and in particular, styrene based thermoplastic elastomer is used preferably.

As a method for preparing the heated mixture, for example, an internal mixer such as a twin-screw extruder, a Banbury mixer, and a kneader is used, and as the condition of mixing by heating, for example, mixing is performed while heating to about 150 to about 250° C. is performed. The method for forming the intermediate layers by using the above-described heated mixture is not subject to any special restriction, and the intermediate layers can be formed, for example, by injection molding or compression molding. In the case in which the injection molding method is used, a method can be used in which after a solid core prepared in advance has been placed at a predetermined position in a mold for injection molding, the above-described material is introduced into the mold. Also, in the case in which the compression molding method is used, a method can be used in which a pair of half cups are prepared using the above-described material, the core is wrapped with these cups directly or via the intermediate layers, and pressure and temperature are applied in the mold. In the case in which molding is performed under pressure and temperature, as the molding conditions, the conditions of about 120 to about 170° C. and about 1 minute to about 5 minutes can be adopted.

To the first and second intermediate layers 20 a and 20 b, a filler or a foaming agent can be added optionally in addition to the above-described principal components of rubber, thermoplastic elastomer, and ionomer resin. As the filler, although not limited to this, for example, barium sulfate, titanium oxide, zinc oxide, and tungsten can be used. The filler is preferably of a powder form. As the blending amount of the filler, although not limited to this, for example, with respect to 100 weight parts of principal component, about 2 weight parts or more is preferable, about 5 weight parts or more is further preferable, and about 10 weight parts or more is still more preferable. Also, the blending amount of the filler is preferably about 1000 weight parts or less, more preferably about 900 weight parts or less, and still more preferably about 800 weight parts or less.

As the foaming agent, although not limited to this, for example, azodicarbonamide, azobisisobutyronitrile, dinitrosopentamethylenetetramine, p-toluenesulfonylhydrazide, p,p′-oxybis (benzene sulfonyl hydrazide), and sodium hydrogen carbonate can be used. As the blending amount of the foaming agent, although not limited to this, for example, with respect to 100 weight parts of principal component, about 2 weight parts or more is preferable, and about 3 weight parts or more is more preferable. Also, the blending amount of foaming agent is preferably about 30 weight parts or less, and more preferably about 25 weight parts or less.

The intermediate layers 20 consisting of the first and second intermediate layers 20 a and 20 b may be formed so as to have a space that is not filled with a material. Such a space, for example, may be a concave-shaped space formed depressing a part of layer surface, or may be a hole-shaped space penetrating the layer. Alternatively, a space can be provided in the intermediate layers 20 by adding a foaming agent to the material of the intermediate layers 20. As the foaming agent, the above-described foaming agent can be used. As the blending amount of the foaming agent, although not limited to this, for example, with respect to 100 weight parts of principal component, about 2 weight parts or more is preferable, and about 3 weight parts or more is more preferable. Also, the blending amount of the foaming agent is preferably about 30 weight parts or less, more preferably about 25 weight parts or less.

The bulk specific gravity of the cover 30 is, although not limited to this, preferably about 0.91 or greater, and more preferably about 0.93 or greater. Also, the bulk specific gravity of the cover 30 is preferably about 1.5 or less, and more preferably about 1.4 or less. As the material for the cover 30, although not limited to this, ionomer resin, polyurethane based thermoplastic elastomer, thermosetting polyurethane, or a mixture of these materials can be used.

The thickness of the cover 30 is, although not limited to this, preferably about 0.2 mm or greater, and more preferably about 0.4 mm or greater. Also, the thickness of the cover 30 is preferably about 4 mm or less, more preferably about 3 mm or less, and still more preferably about 2 mm or less.

On the surface of the cover 30, multiple dimples 32 are formed. The number of dimples 32 on the entire surface of the golf ball 1 is preferably about 200 or greater, more preferably about 250 or greater, and still more preferably about 300 or greater. Also, the upper limit of the number of dimples 32 is preferably about 500, more preferably about 450, still more preferably about 430, and yet still more preferably about 410. By making the number of dimples 32 in this range, the golf ball 1 is made so that it is easy to receive lift, and especially the carry at the time when a driver is used can be increased.

The dimple occupancy ratio, specifically, the ratio of the total of dimple areas defined by a planar surface edge surrounded by the dimple edge to the ball spherical area assumed that the dimples do not exist (the SR value) is preferably about 60% or more, more preferably about 65% or more, and still more preferably about 68% or more from the viewpoint of being capable of achieving the aerodynamic characteristics sufficiently. The upper limit of the dimple occupancy ratio is, although not limited to this, preferably about 90%, more preferably about 85%, and still more preferably about 80%.

The value V₀ obtained by dividing the space volume of a dimple in a flat plane surrounded by the edge of each dimple by the volume of a column whose bottom surface is the above-described flat plane and whose height is the maximum depth of dimple from this bottom surface is preferably about 0.35 or greater from the viewpoint of making the ball trajectory proper. The upper limit of V₀ is, although not limited to this, suitably about 0.80, for example. The VR value, which is a ratio of the total volume of dimples formed on the lower side of the flat plane surrounded by the dimple edge to the ball spherical volume assumed that the dimples do not exist, is preferably about 0.6% or more, more preferably about 0.65% or more, and still more preferably about 0.7% or more. The upper limit of the VR value is preferably about 1.0%, and more preferably about 0.9%.

Multiple kinds of dimples 32 having different diameters and/or depths can be formed. The number of kinds of dimples is preferably 3 or more, more preferably 4 or more, and still more preferably 5 or more. The upper limit of the number of kinds of dimple is preferably about 20, more preferably about 15, and still more preferably about 12. By making the number of dimple kinds in this range, the surface occupancy ratio of dimples is made easy to increase, so that the carry can be increased.

The shape of the dimple 32 can be made a planar circular shape, a planar noncircular shape, or a combination of these shapes. The average diameter of the planar circular-shaped dimple is preferably about 2.8 mm or more, more preferably about 3.5 mm or more, and still more preferably about 3.8 mm or more. The upper limit of the average diameter is preferably about 5.0 mm, more preferably about 4.6 mm, and still more preferably about 4.3 mm. The average depth of the dimple 32 is preferably about 0.120 mm or more, more preferably about 0.130 mm or more, and still more preferably about 0.140 mm or more from the viewpoint of obtaining a proper trajectory. The upper limit of the average depth is preferably about 0.185 mm, more preferably about 0.180 mm, and still more preferably about 0.174 mm.

The average diameter is the average value of diameters of all the dimples, and the average depth is the average value of the depths of all the dimples. In many cases, the golf ball is painted, and therefore the diameter and depth of dimple are measured in a state of being painted. The dimple diameter is measured by measuring the width across between the points at which the land part, which is the golf ball surface on which the dimples do not exist, is in contact with the concave surface of dimple. Also, the dimple depth is measured by measuring the vertical distance from the center position of an imaginary planar circle, which is drawn by connecting the points at which the dimple is in contact with the land part, to the bottom surface of dimple.

The weight of the golf ball 1 is 45.93 g or less in conformity with the golf rules, but is preferably about 45.70 g or less. The lower limit of the weight of the golf ball 1 is not subject to any special restriction; however, it is preferably about 44.00 g, and more preferably about 44.50 g. The diameter of the golf ball 1 is 42.67 mm or greater in conformity with the golf rules. The upper limit of the diameter of the golf ball 1 is not subject to any special restriction; however, it is preferably about 43.00 mm, and more preferably about 42.80 mm.

According to the present invention, as described above, the bulk specific gravity of a spherical zone having a radius R of 18.5 mm with the center point C of the golf ball 1 being the center is made very small, being about 0.7 or smaller, so that the moment of inertia of the golf ball 1 is increased. Therefore, the spin decay rate of the golf ball during flight becomes low, so that the carry can be increased even in the case in which the initial spin speed is low. In FIG. 1, with the position of the radius R of 18.5 mm from the center point C being the boundary, the first intermediate layer 20 a is disposed on the center side thereof, and the second intermediate layer 20 b is disposed on the outside thereof. However, the present invention is not limited to this configuration. If the bulk specific gravity of the above-described zone having the radius R of 18.5 mm and the moment of inertia or the spin decay rate of the golf ball 1 are within the above-described predetermined range, the boundary line between the first intermediate layer 20 a and the second intermediate layer 20 b may be arranged on the center side of the position 18.5 mm distant from the center point C, or may be on the outside thereof. Preferably, the boundary line is at a position about 17.0 mm to 20.0 mm distant from the center point C. In particular, to facilitate the optimization of ball weight, the boundary line is preferably at the position 18.5 mm distant from the center point C. Also, each of the first and second intermediate layers 20 a and 20 b is not limited to a single layer as shown in FIG. 1, and can be provided with a plurality of layers.

EXAMPLES

Golf balls having the configurations given in Table 1 were prepared, and the tests for measuring the moment of inertia, the carry, and spin speed of the golf ball, and the durability tests were conducted. The test results are given in Table 1. The blends A to H (wt %) of materials for the core and the first and second intermediate layers given in Table 1 are given in Table 2. The blends I to M (wt %) of materials for the core, the first and second intermediate layers, and the cover are given in Table 3. In working examples 1 to 3 and comparative examples 2 to 4, the central portion of the core was made of a hollow structure. In comparative example 1, the core is of a solid structure and the intermediate layer consists of a single layer. The configuration of this single intermediate layer was described in the column of the second intermediate layer in Table 1 for ease of comparison. Also, in all working examples and comparative examples, the arrangement of dimples was made the same. Specifically, nine kinds of dimples were used, and the number of dimples was made 336, the average diameter of a dimple was made 4 mm, and the average depth of a dimple was made 0.161 mm.

TABLE 1 Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 1 Example 2 Example 3 Example 4 Core Center Outer diameter (mm) 3.0 3.0 3.0 37.0 15.0 20.0 3.0 part Bulk specific gravity 0 0 0 1.20 0 0 0 Weight (g) 0 0 0 31.8 0 0 0 Blend — — — C — — — Surrounding Outer diameter (mm) 33.0 13.0 13.0 — 21.0 24.0 33.0 layer Bulk specific gravity 0.45 1.30 2.00 — 1.30 1.10 0.94 Weight (g) 8.5 1.5 2.3 — 4.0 3.4 17.7 Blend I A B — A D J First Outer diameter (mm) 37.0 37.0 37.0 36.0 38.6 37.0 intermediate Bulk specific gravity 0.94 0.45 0.45 0.94 0.94 0.94 layer Weight (g) 7.2 11.4 11.4 18.4 21.4 7.2 Blend J I I J J J Second Outer diameter (mm) 40.0 40.0 40.0 40.0 40.0 40.0 40.0 intermediate Bulk specific gravity 2.90 3.30 3.20 0.94 1.50 3.20 1.93 layer Weight (g) 20.3 23.1 22.4 6.6 13.6 11.2 13.5 Blend E F G J K G H Cover Outer diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Bulk specific gravity 1.30 1.30 1.30 0.97 1.30 1.30 0.97 Weight (g) 9.4 9.4 9.4 7.0 9.4 9.4 7.0 Blend L L L M L L M Golf ball Weight (g) 45.4 45.4 45.5 45.4 45.5 45.4 45.4 Moments of inertia 101 101 99 80 90 97 88 BSG_(R≦18.5) 0.59 0.49 0.52 1.20 0.92 0.81 0.94 Durability ◯ ◯ ◯ ◯ X X ◯ Distance (m) 8 deg 1.3 1.3 1.2 — 0.6 1.1 0.4 11 deg 1.5 1.4 1.3 — 0.7 1.2 0.5 Spin decay rate [%] 2.5 2.5 2.6 3.3 2.9 2.7 3.0

TABLE 2 A B C D E F G H BR730 100 100 100 100 100 100 100 100 Dicumyl 1 1 1 1 1 1 1  1 peroxide Zinc oxide 40.9 178.4 24.8 4.6 463.9 1226.1 557.6 16.3. Nocrac 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 NS-6 Zinc 20.0 20.0 20.0 5.0 25.5 20.0 36.0 25.5 acrylate Tungsten — — — 10.0 — — 36.4 —

BR730 is the trade name of 1,4-cis-polybutadiene available from JSR Corporation, which was used as the base rubber.

Dicumyl peroxide is available from NOF Corporation, which was used as the initiator.

Zinc oxide is available from Sakai Chemical Industry Co., Ltd.

Nocrac NS-6 is the trade name of 2-2′-methylenebis(4-methyl-6-t-butylphenol) available from Ouchi Shinko Chemical Industry Co., Ltd., which was used as an antioxidant.

Zinc acrylate is available from Nihon Jyoryu Kogyo Co., Ltd.

Tungsten is available from Nippon Tungsten Co., Ltd. (in powder form).

TABLE 3 I J K L M Dynaron 6100P — 32 — — — Polytail H —  2 — — — Behenic acid — 18 — — — Calcium hydroxide —   2.3 — — — Foaming agent  4 — — — — Trimethylolpropane   1.1 — — — — Himilan 1706 35 — — — — Himilan 1557 15 — 75 75 75 Himilan 1605 50 66 — — — Himilan 1855 — — 25 25 25 Magnesium stearate — —   1.8   1.8   1.8 Titanium oxide — —   3.8   3.8   3.8 Barium sulfate 300 — —   77.2   48.1 —

Dynaron 6100P is the trade name of a hydrogenated thermoplastic elastomer available from JSR Corporation.

Polytail H is the trade name of polyolefin polyol available from Mitsubishi Chemical Corporation.

Behenic acid is available from NOF Corporation.

Calcium hydroxide is available from Shiraishi Kogyo Kaisha Ltd.

The foaming agent is the ADCA master batch available from Otsuka Chemical Co., Ltd.

Himilan 1706 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

Himilan 1557 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

Himilan 1605 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

Himilan 1855 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

Barium sulfate 300 is available from Sakai Chemical Industry Co., Ltd.

The moment of inertia of the golf ball was measured by using a measuring instrument for moment of inertia (M01-005 available from Inertia Dynamics Inc.). This measuring instrument calculates the moment of inertia of a golf ball from a difference between the vibration period at the time when the golf ball is placed on a jig of the measuring instrument and the vibration period at the time when the golf ball is not placed thereon.

The durability of the golf ball was evaluated by using an ADC Ball COR Durability Tester available from Automated Design Corporation in U.S.A. This tester has the function of discharging a golf ball by using a pneumatic pressure and thereafter causing the golf ball to collide continuously with two metal plates disposed in parallel with each other. The test results were evaluated as in Table 1 so that a circle mark indicates that the number of discharges required until the ball is broken is 30 or more, a triangle mark indicates that the number of discharges is less than 30 and not less than 20, and a cross mark indicates that the number of discharges is less than 20. As the measurement conditions, the incidence velocity of the ball was made 43 m/s.

The carry of the golf ball was tested by using a launcher (UBL available from Automated Design Corporation) under the conditions that the ball initial velocity was 59 m/s, the initial spin speed was 2000 rpm, and the delivery angles were 8 degrees and 11 degrees. The result of carry test in Table 1 is shown by the increased distance (m) of each example with the carry of comparative example 1 being the reference. The UBL is a device in which two pairs of drums are installed vertically, and belts are set around the upper two drums and the lower two drums, the drums are rotated and a ball is inserted therebetween, whereby the ball is launched under desirable conditions.

The spin speed of the golf ball during flight with respect to time was measured by using a golf ball trajectory tracking system (TrackMan available from TrackMan A/S). From these measurement data, the gradient of an approximated straight line was calculated by the least-squares method, and the spin loss per one second of the golf ball was determined. The spin decay rate was defined as a value obtained by dividing the spin loss per one second by the initial spin speed.

As shown in Table 1, as compared with the golf ball of comparative example 1 of a conventional construction provided with a solid core having an outside diameter of 37 mm and a bulk specific gravity of 1.20, an intermediate layer having an outside diameter of 40 mm and a bulk specific gravity of 0.94, and a cover having an outside diameter of 42.7 mm and a bulk specific gravity of 0.97, in working example 1, the core has a hollow part having an outside diameter of 3.0 mm, the surrounding layer having an outside diameter of 33.0 mm has a bulk specific gravity of 0.45, and the first intermediate layer having an outside diameter of 37 mm has a bulk specific gravity of 0.94. Thereby, the bulk specific gravity of the spherical zone having a radius R of 18.5 mm (hereinafter, referred to as the “BSG_(R≦18.5)”) could be decreased significantly from 1.20 to 0.59. Also, since the bulk specific gravity of the second intermediate layer having an outside diameter of 40 mm was made 2.90, the moment of inertia could be increased greatly from 80 g·cm² to 100 g·cm². As a result, the spin decay rate of working example 1 decreased to 2.5% as compared with 3.3% of comparative example 1, and also the carry increased by 1.3 to 1.5 m as compared with comparative example 1.

For the golf ball of working example 2, in which as compared with working example 1, the outside diameter of the surrounding layer is decreased to 13.0 mm, the bulk specific gravity of the surrounding layer is increased to 1.30 and, on the other hand, the bulk specific gravity of the first intermediate layer is decreased to 0.45, and the bulk specific gravity of the second intermediate layer is increased to 3.30, the BSG_(R≦18.5) could be decreased to 0.49, and the moment of inertia could be increased to 101 g·cm². As a result, the spin decay rate of working example 2 decreased to 2.5% as compared with 3.3% of comparative example 1, and also the carry increased by 1.3 to 1.4 m as compared with comparative example 1.

For the golf ball of working example 3, in which bulk specific gravity of the surrounding layer is more increased to 2.00 and, on the other hand, the bulk specific gravity of the second intermediate layer is slightly decreased to 3.20, the BSG_(R≦18.5) could be decreased to 0.52, and the moment of inertia could be increased to 99 g·cm². As a result, the spin decay rate of working example 3 decreased to 2.6% as compared with 3.3% of comparative example 1, and also the carry increased by 1.2 to 1.3 m as compared with comparative example 1.

For the golf ball of comparative example 2, in which as compared with working example 1, the outside diameter of the hollow part is increased to 15.0 mm, the outside diameter of the surrounding layer is decreased to 21.0 mm, the bulk specific gravity of the surrounding layer is increased to 1.30, the outside diameter of the first intermediate layer is slightly decreased to 36.0 mm, and the bulk specific gravity of the second intermediate layer is decreased to 1.50, although the BSG_(R≦18.5) was 0.92, being smaller than that of comparative example 1, it was far larger than those of working examples 1 to 3. Also, although the moment of inertia was 90 g·cm², being higher than that of comparative example 1, it was lower than those of working examples 1 to 3. As a result, the spin decay rate of comparative example 2 was as high as 2.9%, and the carry increased merely by 0.6 to 0.7 as compared with comparative example 1. Also, the golf ball of comparative example 2 has a problem of poor durability because the outside diameter of the hollow part thereof is large.

For the golf ball of comparative example 3, in which as compared with comparative example 2, the outside diameter of the hollow part is more increased to 20.0 mm, the outside diameter of the surrounding layer is increased to 24.0 mm, the bulk specific gravity of the surrounding layer is decreased to 1.10, the outside diameter of the first intermediate layer is increased to 38.6 mm, and the bulk specific gravity of the second intermediate layer is significantly increased to 3.20, although the BSG_(R≦18.5) was 0.81, being smaller than that of comparative example 2, it was still larger than those of working examples 1 to 3. Although the moment of inertia of the golf ball of comparative example 3 was as high as 97 g·cm², the spin decay rate thereof was as low as 2.7%, and the carry thereof also increased by 1.1 to 1.2 m as compared with comparative example 1, the golf ball of comparative example 3 has a problem of poor durability because the outside diameter of the hollow part thereof is large.

For the golf ball of comparative example 4, in which as compared with working example 1, the bulk specific gravity of the surrounding layer is increased to 0.94 and, on the other hand, the bulk specific gravity of the second intermediate layer is decreased to 1.93, and the bulk specific gravity of the cover is decreased to 0.97, although the BSG_(R≦18.5) was 0.94, being smaller than that of comparative example 1, it was still larger than those of working examples 1 to 3. Also, although the moment of inertia was 88 g·cm², being higher than that of comparative example 1, it was lower than those of working examples 1 to 3. As a result, the spin speed of comparative example 4 is as high as 3.0%, and the carry is also increased merely by 0.4 to 0.5 as compared with comparative example 1. 

What is claimed is:
 1. A multiple-piece golf ball comprising: a core located in a center of the golf ball; at least two intermediate layers surrounding the core; and a cover further surrounding the intermediate layers, wherein the golf ball has a spherical zone having a bulk specific gravity of up to about 0.7, the spherical zone having a radius of 18.5 mm with the same center point as the golf ball.
 2. The golf ball according to claim 1, wherein the core includes a hollow part in a center thereof and a surrounding layer surrounding the hollow part.
 3. The golf ball according to claim 2, wherein at least either one of the innermost intermediate layer of the at least two intermediate layers and the surrounding layer has a bulk specific gravity of up to about 0.8.
 4. The golf ball according to claim 2, wherein the hollow part has a spherical shape having a diameter smaller than about 5 mm.
 5. The golf ball according to claim 2, wherein the surrounding layer is a foam.
 6. The golf ball according to claim 2, wherein an intermediate layer located in the spherical zone of the at least two intermediate layers is a foam.
 7. The golf ball according to claim 2, wherein the surrounding layer has a space not filled with a material.
 8. The golf ball according to claim 2, wherein an intermediate layer located in the spherical zone of the at least two intermediate layers has a space not filled with a material.
 9. The golf ball according to claim 1, wherein the core is a foam.
 10. The golf ball according to claim 1, wherein the core has a space not filled with a material. 